Today's electronic devices are built by connecting together electrical components, ranging from a few electrical components for more simple circuits to millions of them for more complex circuits. Low power has become one of the main issues in electronics industry for many product areas such as cellular telephones, biomedical implants, digital watches, calculators, tape players, portable computers, in short all kinds of portable and battery powered electronic devices.
There where household electronic devices that are powered by mains voltage can tolerate a certain power loss in the electronic circuits, battery powered devices cannot. For these devices, this power loss has a direct connection with the battery life, and a shorter battery life directly impacts the usability of the device.
Power loss in a circuit is among other things dependent upon the supply voltage. Reducing the supply voltage of an electronic circuit requires a reduction of the threshold voltages of the MOS devices in the circuit. It is known that MOSFET sub-threshold current, which is the transistor current if the transistor is biased below the threshold voltage, increases exponentially as threshold voltage is reduced. Therefore MOSFET devices with low threshold voltages generally have relatively large leakage currents. In order to avoid this leakage current problem up to now the threshold voltage of all transistors in a circuit is made sufficiently high to prevent leakage. Furthermore, in a basic CMOS (Complementary Metal Oxide Semiconductor) bulk process for making electronic circuits, n-MOS and p-MOS transistor threshold voltages are chosen as symmetrical as possible for digital circuit considerations, in order to minimise the static circuit consumption, i.e. minimisation of the off-state current. Analog performance can be improved if transistors with different threshold voltages are available, mainly for dynamic voltage range purposes. Even n-MOS transistors with a negative threshold voltage or p-MOS transistors with a positive threshold voltage can be very useful. But even then, if such n-MOS and p-MOS transistors are used, they have symmetrical threshold voltages.
Different kinds of electronic low power components or low power circuits are known, which use standard CMOS components coupled to each other in a particular way in order to minimise power loss.
U.S. Pat. No. 5,506,527 describes a low power diode having a comparator for comparing the voltage present at the anode and the cathode of the diode. When the comparator determines that the voltage present at the anode of the low power diode equals or exceeds the voltage present at the cathode of the low power diode by a predetermined forward voltage, a signal is generated. This signal turns on a transistor acting as a switch, which in turn electronically connects the anode and the cathode of the low power diode together. Unlike conventional diodes that have a forward voltage of approximately 0.7 Volts (depending on the physical silicon junction property of the diode), the described low power diode has a very small forward voltage of approximately 0.25 Volts (depending on the drain to source resistance of the switch when on and on the offset of the comparator). This diode thus has a low threshold voltage and at the same time a small leakage current.
U.S. Pat. No. 4,860,257 describes a level-shift circuit with large sized p-MOS transistors and small sized n-MOS transistors. Low power dissipation in this case results from the fact that the current conducting circuit is connected through very small transistors when the clock is active, and that the current-conducting circuit disappears when the clock is not active.
It is an object of the present invention to provide an ultra-low power (ULP) device with a low threshold and at the same time low leakage current, which ULP device can be used as a basic block in low power applications.